1. Field of the Invention
The present invention relates to the field of engine control and more particularly to the combustion control of spark-ignition engines.
2. Description of the Prior Art
Operation of a (gasoline type) spark-ignition engine is based on the combustion of a mixture of air, burnt gas and fuel. The engine cycle can be broken down into four phases (FIG. 1):
The intake phase (ADM): the intake valve allows the mixture of air and of burnt gas into chamber CHB. The air is taken from the outside environment of the engine. The burnt gas is taken from exhaust manifold ECH and sent back to the intake manifold (exhaust gas recirculation EGR) and/or sucked back by the exhaust valve (internal exhaust gas recirculation iEGR). The fuel is injected during the intake phase. The Variable Valve Timing (VVT) device allows a time lag to be applied to the intake (VVTa) and exhaust (VVTe) valve lift profiles. This has a direct impact on the gas composition and on the turbulence in the combustion chamber;
The compression phase: after the intake valve closes (IVC: Intake Valve Closing), piston PIS compresses the gas;
The combustion phase: spark plug BOUG produces a spark that initiates the combustion of the mixture of air, burnt gas and fuel which ignites while releasing the chemical energy available in the fuel, thus creating an overpressure that pushes the piston backwards;
The expansion phase: once the piston has gone back down again, the exhaust valve opens and the gas mixture is then discharged through the exhaust manifold.
The goal of engine control is to supply the driver with the torque required while minimizing the noise and pollutant emissions. Control of the amounts of the different gases and of the fuel therefore has to be adjusted as finely as possible.
To carry out combustion control of a spark-ignition engine, there are known methods allowing determination of the combustion medium by means of detectors mounted in the engine. There are methods based on direct measurements in the combustion chamber, such as those provided by:
Cylinder Pressure Detectors: Paljoo Yvon et al., “Closed-loop Control of Spark Advance and Air-fuel Ratio in SI Engines Using Cylinder Pressure”, Society of Automotive Engineering World Congress, 2000-01-0933,
Ionization Current Detectors in the Combustion Chamber: Lars Eriksen et al., “Closed loop Ignition Control by Ionization Current Interpretation”, SAE 1997 Transactions, Journal of Engines, Vol. 106, Section 3, pp. 1216-1223, 1997.
However, using such detectors in standard vehicles is difficult due to their considerable cost. Furthermore, these detectors are generally subject to relatively fast drifts.
There are also methods wherein the amounts and timing are optimized on each static working point (speed and torque) so as to bring out an ideal strategy at each point. An engine test bench calibration is therefore performed in order to obtain the optimum values for the main three data sets:
Air Loop:
the mass of air Mair and of burnt gas Mgb required in the combustion chamber;
the pressure and the temperature of these gases in the combustion chamber; and
the position of the variable valve lift devices (VVT) and notably the intake valve closure angle denoted by θivc.
These thermodynamic and physical variables Xair=(Mair, Mbg, P, T, θIVC) are represented by Xair.
Fuel Loop:
The mass of fuel Mf injected into the combustion chamber (injection directly into the chamber or indirectly into the intake pipe), Xfuel=(Mf).
Ignition Loop:
The crank angle θall at which the spark appears (via the plug), denoted by Xall=(θall).
However, these strategies are insufficient in transient phases. In fact, during transition phases from one working point to another (change in the vehicle speed or in the road profile), the engine control supervises the various actuators present in the engine to guarantee the desired torque while minimizing the noise, the pollutant emissions and the consumption. This is thus translated into the change from the values of the parameters of the initial point to the values of the parameters of the final point:
                    {                                                                              X                  air                  initial                                →                                  X                  air                  final                                                                                                                          X                  fuel                  initial                                →                                  X                  fuel                  final                                                                                                                          X                  all                  initial                                →                                  X                  all                  final                                                                                                                                                                                  (                    a                    )                                                                                                                    (                    b                    )                                                                                                                          (              c              )                                          
Now, there are two time scales in the engine. The faster one (50 Hz) corresponds to the entire combustion phenomenon (1 engine cycle). On this scale, the injection (Xfuel) and the ignition (Xall) strategy can be changed to control the combustion. The slower one (1 Hz) corresponds to the gas dynamics in the engine manifolds (intake, exhaust, burnt gas recirculation) and the inertia of the actuators (turbocompressor TC). The strategy of this air loop (Xair) cannot be changed faster.
With current methods, the controlled variables (Xair, Xfuel, Xall) do therefore not reach at the same time their setpoint values because of the difference in dynamics. The objectives regarding torque production, namely, consumption, pollutants, and noise are thus met in the static phases (the dynamic loops are stabilized at their reference values). On the other hand, if precautions are not taken in the transient phases, part of the parameters reach nearly instantaneously their final setpoint value whereas the other part is still at the initial setpoint value. This results in the engine then producing more pollutant emissions or noise and can even cause stopping in some cases.
Furthermore, without cylinder pressure detectors, the known methods do not allow controlling combustion timing during the transient phases. Now, as illustrated by FIGS. 2 and 3, this is not sufficient to ensure proper operation of the engine under transient conditions.